Olefin based polymers such as polyethylene are produced via various catalyst systems and polymerization processes. Selection of such catalyst systems used in the polymerization process of the olefin based polymers is an important factor contributing to the characteristics and properties of such olefin based polymers.
Polyethylene is known for use in the manufacture of a wide a variety of articles. The polyethylene polymerization process can be varied in a number of ways to produce a wide variety of resultant polyethylene resins having different physical properties suitable for use in different applications. It is generally known that polyethylene can be produced in solution phase loop reactors in which ethylene monomer, and optionally one or more alpha olefin comonomers, typically having from 3 to 10 carbon atoms, are circulated in the presence of one or more catalyst systems under pressure around a loop reactor by a circulation pump. The ethylene monomers and optional one or more comonomers are present in a liquid diluent, such as an alkane or isoalkane, for example isobutane. Hydrogen may also be added to the reactor. The catalyst systems for producing polyethylene may typically comprise a chromium-based catalyst system, a Ziegler Natta catalyst system, and/or a molecular (either metallocene or non-metallocene) catalyst system. The reactants in the diluent and the catalyst system are circulated at an elevated polymerization temperature around the loop reactor thereby producing polyethylene homopolymer and/or copolymer depending on whether or not one or more comonomers are present. Either periodically or continuously, part of the reaction mixture, including the polyethylene product dissolved in the diluent, together with unreacted ethylene and one or more optional comonomers, is removed from the reactor (e.g. loop reactors, isothermal reactors, fluidized bed reactors, stirred tank reactors, batch reactors in parallel, series, and/or any combinations thereof). The reaction mixture when removed from the loop reactor may be processed to remove the polyethylene product from the diluent and the unreacted reactants, with the diluent and unreacted reactants typically being recycled back into the loop reactor. Alternatively, the reaction mixture may be sent to a second reactor, e.g. loop reactor, serially connected to the first loop reactor where a second polyethylene fraction may be produced.
Despite the research efforts in developing catalyst systems and processes suitable for polyolefin polymerization, there is still a need for an improved polyolefin polymerization process reducing the production of any undesired oligomers.